Hi joinfof,
You’d probably want to start by looking at our Themer’s Guide – what you’d be interested in here of course are the templates relating to photo view.
In this case you would probably be particularly interested in pro/photo/single-event.php and the call to tribe_event_featured_image() which takes place close to the start of that particular template. You could build your own conditional statement to control when it is used, and might for instance build on the following functions to figure out if an events category is being used and, if so, which one:
Hope that helps!